MessageBox: fixed issue where it is possible for the button in the click event handler to be null.

Main/Source/ExtendedWPFToolkitSolution/Src/WPFToolkit.Extended/MessageBox/Implementation/MessageBox.cs

@@ -512,6 +512,10 @@ namespace Microsoft.Windows.Controls
         private void Button_Click(object sender, RoutedEventArgs e)
         {
             Button button = e.OriginalSource as Button;
+
+            if (button == null)
+                return;
+
             switch (button.Name)
             {
                 case "PART_NoButton":